The present invention relates to a positional information transmitter, a positional information receiver and a position measuring system which acquire the current position of each portable terminal indoors or the like, where radio waves sent from satellites cannot be caught.
In a system which performs positioning using wireless signals (hereinafter called “satellite positioning signals”) transmitted from satellites such as a Global Positioning System (GPS) satellite, etc., the accuracy of positioning is degraded or positioning is disabled where portable terminals such as GPS receivers or the like are located in areas such as indoors, underground malls, etc., where the satellite positioning signals cannot be received.
As solutions to the above, there has been disclosed in, for example, Japanese Unexamined Patent Publication No. 2007-278756, a technology wherein positional information transmitters (indoor transmitters) which respectively transmit positional information being information indicative of positions in the form of signals (hereinafter called “positional information signals”) each compatible with a frequency (e.g., center frequency 1.57542 GHz), a modulation system (specifically, Binary Phase-Shift Keying (BPSK)), a multiple access method (specifically, Code Division Multiple Access (CDMA) of direct-spectrum diffusion method), etc. used in GPS, are installed in areas such as indoors, underground malls, etc., where satellite positioning signals cannot be received, and portable terminals such as cellular phones, etc. acquire their own current positions from the received positional information signals. Further, there has been disclosed in Japanese Unexamined Patent Publication No. 2009-133731, a technology that shortens the time taken until the acquisition of positional information.
Such positioning using the positional information signals as described above is different from the positioning based on the satellite positioning signals. Each portable terminal takes a position included in the received positional information signals as its own current position as it is without performing complex positioning calculations. Therefore, the positional information transmitters are installed at intervals corresponding to the required accuracy of positioning and adjust their outputs in such a manner that the transmitted positional information signals are not detected from faraway places. For example, when the positional information transmitters are installed at 10 m intervals, their outputs are adjusted in such a manner that the range of detection of each positional information signal falls from 10 m to 15 m in radius.
Thus, since many positional information transmitters are instanced adjacent to one another in the case of the positioning using the positional information signals, it is desirable that Pseudo Random Noise (PRN) numbers for selecting code patterns used in spectrum diffusion are prevented from overlapping between the adjacent positional information transmitters in order to avoid interference between the positional information signals. The number of PRN numbers available indoors is, however, limited to be 10 or so in advance.
However, when positional information signals transmitted from positional information transmitters installed in the ceilings lying indoors and in underground malls are received by, for example, a portable terminal held at about human chest height, multipath occurs due to interference with reflected waves from the floor surface and the like. At this time, multipath fading during which the received signals become extremely weak, occurs where the portable terminal is moving, thereby causing burst bit errors.
FIGS. 11 and 12 are respectively image diagrams each showing the relationship between the distance from a transmission source and received power at the time that a portable terminal 93 using a prior art is moved while receiving radio waves (in a 1.57542 GHz band) of positional information signals each compatible with a GPS satellite positioning signal.
As shown in FIG. 11, a large number of positional information transmitters 92 (only one is described as one example in FIG. 11) are installed in the ceiling planes lying indoors or the like where the satellite positioning signals cannot be received, at intervals corresponding to the required accuracy of positioning. The positional information transmitter 92 includes antennas 930 provided more than at least one. The positional information transmitter 92 transmits a positional information signal PS91 which includes positional data indicative of the position of installation thereof and is compatible with the satellite positioning signal, from the antenna 930. The speed of the positional information signal PS91 is slow at 50 bps. The transmitted positional information signal PS91 is spectrum-diffused according to code patterns of PRN numbers and modulated to a carrier of a predetermined frequency band, followed by being transmitted.
Described concretely, a positional information signal PS91 spectrum-diffused (encoded) in a code pattern relative to a PRN number 173 and including positional data (x1, y1, z1) indicative of the longitude, latitude and altitude is transmitted from the corresponding antenna 930 of the positional information transmitter 92. When the positional information transmitter 92 has a plurality of antennae 930, positional information signals transmitted from the respective antennas 930 respectively use PRN numbers different from those for satellite positioning (i.e., they are encoded by different code patterns), thus causing no interference between the positional information signals. Thus, when a plurality of positional information transmitters 92 exist within the range of radio wave propagation, a portable terminal 93 receives positional information signals corresponding to a plurality of PRN numbers.
If the plural PRN numbers are PRN numbers utilized for indoor positioning where the portable terminal 93 has received the positional information signals corresponding to the PRN numbers, then the portable terminal 93 selects only one having the maximum received signal strength out of all the positional information signals received and measures its own position as (x1, y1, z1) from positional data contained in the selected positional information signal.
However, even when the positional information signal corresponding to the PRN number 173 is transmitted to the portable terminal 93, for example, the positional information signal may not be received properly. When a positional information signal PS91 indicative of the latitude, longitude and altitude (x1, y1, z1) is received from an antenna 930 of a positional information transmitter 92 as shown in FIG. 12, for example, normally, frames corresponding to the four are taken, 120 bits in total are taken as the amount of data, and a time of about 2.4 seconds is taken. Thus, when the portable terminal 93 is moving, it falls into valleys of received power due to multipath fading at the timing at which the received power of the portable terminal 93 falls suddenly. Since the positional information signal encounters burstwise bit errors in this case, the portable terminal 93 is not able to receive the positional information signal.
Thus, the prior art is accompanied by the problems that when, for example, the portable terminal moves over the range of detection of each positional information signal, having a radius ranging from about 10 m to about 15 m at about 4 km/h, burstwise bit errors occur due to the effects of multipath fading even if the positional information signal transmitted from the positional information transmitter 92 cannot be captured or vice versa, and hence the position may not be measured appropriately while the portable terminal is being moved without the positional information being able to get.